Line tension in a thick soap film

TL;DR

This paper demonstrates that line tension influences the formation of circular patterns in soap films, with measurements indicating a line tension of about 0.1 nN that affects film relaxation and stability.

Contribution

It provides the first quantitative measurement of line tension in thick soap films and links it to pattern formation and film stability.

Findings

Line tension measured at approximately 0.1 nN.

Line tension drives the relaxation of elongated film shapes.

Line tension influences soap film drainage and stability.

Abstract

The thickness of freshly made soap films is usually in the micron range, and interference colors make thickness fluctuations easily visible. Circular patterns of constant thickness are commonly observed, either a thin film disc in a thicker film or the reverse. In this Letter, we evidence the line tension at the origin of these circular patterns. Using a well controlled soap film preparation, we produce a piece of thin film surrounded by a thicker film. The thickness profile, measured with a spectral camera, leads to a line tension of the order of 0.1 nN which drives the relaxation of the thin film shape, initially very elongated, toward a circular shape.A balance between line tension and air friction leads to a quantitative prediction of the relaxation process. Such a line tension is expected to play a role in the production of marginal regeneration patches, involved in soap film drainage and stability.